The primary objective of the proposed work is to deveop hydrophilic polymeric microspheres suitable to label membrane receptors and to separate cell subpopulations using immunological techniques. The structure of the polymer and microspheres synthesized from monoglutaraldehyde was elucidated by means of I.R. and U.V. spectroscopy. Magnetite particles were incorporated into the polyglutaraldehyde (PGL) microspheres (1000 A in diameter). The latter were covalently bonded to antibodies. IgG bearing lymphocytes were labeled with the magnetic PGL microsphere antibody conjugates and separated from unlabeled cells by means of a magnetic field. The electrophoretic mobility of human red blood cells labeled with PGL microspheres was 40% less than of unlabeled cells. This permitted separation of model cell mixtures by means of a free flow electrophoretic apparatus.